I've only given this a few minutes of thought, but if I don't respond now then I probably never will, and I'd like to at least try to help a little. So here's a few ideas... just kinda tossing them out as they come to mind.
One area I think High Schools have traditionally overlooked, or failed to accomplish, is APPLIED education... by this I mean, relating/applying the material to real-life situations that the student can themselves relate to. The "relation" doesn't even necessarily have to be something that the student is already familiar with. For instance: SL minimally acknowledges imported picture dimensions; therefore, I take the pixel dimensions, (say, 1280x800), calculate the ratio (1280 / 800 = 8:5 or 1.6), and then i can use those numbers to resize two dimensions of a box to accurately display the picture (say, 8 / 4 = 2 meters for X, 5 /4 = 1.25 meters for Y)
A few months ago I met a professor who was looking into SL for a similar purpose, but more on the college level. There's multifarious opportunities for research in SL: computer game design, anthropology, sociology, basic programming, graphic design, business, real estate...
Simply learning how to play SL can be an educating experience. Locations are vectors... if you've never in your life known what a vector is, you WILL gain a general understanding soon after logging into SL for the first time.
In terms of GED and Adult Ed, focus on the separate generalized subject categories that need to be taught, layout some of the more specific sub-categories, and then isolate aspects of SL that can aide each of these subjects/categories. like..
Math - (Geometry, Algebra, etc)
Science - (Biology, Chemistry, Astronomy, etc) * i think this one'll be tough

Literal Arts - (Reading & Writing, Typistry, etc)
etc etc
btw I dunno if "typistry" is a word but, i think you'll get my drift.

ok my brain is empty, time to go bounce my virtual representation on a virtual trampoline for a few hours! YAY!